The superconductivity uncovered in atomically thin two-dimensional
(2D) forms of layered transition metal dichalcogenides (TMDs) have
recently attracted remarkable scientific and technical
interests [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}.
Although these superconductors belong to the conventional \(s\) -wave
superconductivity with low transition
temperature [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]},
the uniqueness of the TMDs makes them alluring to the researchers.
On one hand, similar to graphene, these materials have a honeycomb
lattice structure, and exhibit a valley degree of freedom with
minima/maxima of conduction/valence bands at the corners
\(\mathbf {K}\)  and \(-\mathbf {K}\)  of the Brillouin zone. On the other
hand, unlike graphene, the in-plane mirror symmetry is broken in the
TMDs, leading to a strong atomic Ising type spin-orbital coupling
(ISOC) [3]}, [4]}, [7]}, [8]}, [9]}. The ISOC
strongly pins the electron spins to the out-of-plane directions and
have opposite directions in opposite valleys (\(\mathbf {K}\)  and
\(-\mathbf {K}\) ) [3]}, [4]}, [7]}, [8]}, [9]}, [12]}, [14]}, [41]}, [42]},
so that it preserves time-reversal symmetry and is compatible with
superconductivity. Due to the strong pinning of electron spins in
the out-of-plane directions, external in-plane magnetic fields are
much less effective in aligning electron spins, and lead to the
in-plane upper critical field \(H_{c2}\)  of the system several times
larger than the Pauli limit [10]}, [12]}.
